Honeydew sugars in wild-caught female horse flies (Diptera: Tabanidae)

Three species of horse fly, Hybomitra illota (Osten Sacken), H. affinis (Kirby), and H. zonalis (Kirby), were collected by sweep-netting around human bait at 2 sites (abandoned airfield and Davies Bog) in Algonquin Provincial Park, Ontario. There were 3 times more horse flies collected at Davies Bog than at the abandoned airfield. The crop contents of all specimens were identified by thin-layer chromatography. Using melezitose and stachyose as honeydew indicator sugars, the relative importance of homopteran honeydew and floral nectar as carbohydrate sources for these flies was determined. Of flies testing sugar-positive, 42.0% of H. illota (n = 50), 52.8% of H. affinis (n = 36), and 100% of H. zonalis (n = 4) had recently fed on honeydew. Pooled data for the 3 species showed that 38.9% of flies at the abandoned airfield (n = 18) and 51.4% of flies at Davies Bog (n = 72) were honeydew-fed. There were no significant differences among species or between sites. These findings are compared with recent black fly and deer fly studies from the same sites.